Test return value type

core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php

@@ -7,6 +7,7 @@
 use Drupal\Core\Datetime\DateHelper;
 use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\Core\Language\Language;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -246,6 +247,9 @@ public function testDayOfWeekName(): void {
     $this->assertNotNull(DateHelper::dayOfWeekName(FALSE));
     $this->assertNotNull(DateHelper::dayOfWeekName(''));
 
+    // Ensure proper return value type.
+    $this->assertInstanceOf(TranslatableMarkup::class, DateHelper::dayOfWeekName());
+
     // Pass nothing and expect to get NULL.
     $this->assertNull(DateHelper::dayOfWeekName(0));
     $this->assertNull(DateHelper::dayOfWeekName('0'));